Novel Free Layer Structure in Magnetic Random Access Memory (MRAM) for Mo or W Perpendicular Magnetic Anisotropy (PMA) Enhancing Layer

ABSTRACT

A perpendicularly magnetized magnetic tunnel junction (p-MTJ) is disclosed wherein a free layer (FL) has a first interface with a MgO tunnel barrier, a second interface with a Mo or W Hk enhancing layer, and is comprised of FexCoyBz wherein x is 66-80, y is 5-9, z is 15-28, and (x+y+z)=100 to simultaneously provide a magnetoresistive ratio &gt;100%, resistance x area product &lt;5 ohm/□m2, switching voltage &lt;0.15 V (direct current), and sufficient Hk to ensure thermal stability to 400° C. annealing. The FL may further comprise one or more M elements such as O or N to give (FexCoyBz)wM100-w where w is &gt;90 atomic %. Alternatively, the FL is a trilayer with a FeB layer contacting MgO to induce Hk at the first interface, a middle FeCoB layer for enhanced magnetoresistive ratio, and a Fe or FeB layer adjoining the Hk enhancing layer to increase thermal stability.

PRIORITY DATA

The present application is a divisional application of U.S. patentapplication Ser. No. 15/933,479, filed Mar. 23, 2018, which is hereinincorporated by reference in its entirety.

RELATED PATENT APPLICATIONS

This application is related to the following: U.S. Pat. Nos. 8,372,661;9,472,752; and 9,780,299; which are herein incorporated by reference intheir entirety.

TECHNICAL FIELD

The present disclosure relates to a perpendicularly magnetized magnetictunnel junction (p-MTJ) comprised of a free layer that has a firstinterface with a tunnel barrier layer and a second interface with ametal PMA (Hk) enhancing layer such as Mo or W that increases PMA andthermal stability in the free layer while reducing a resistance x area(RA) product compared with a p-MTJ having a metal oxide Hk enhancinglayer, and optimizing a figure of merit A where A is the product of Hkand the magnetoresistive (MR) ratio in order to satisfy magneticperformance requirements for advanced MRAM products.

BACKGROUND

Perpendicularly magnetized MTJs (p-MTJs) are a major emerging technologyfor use in embedded magnetic random access memory (MRAM) applications,and standalone MRAM applications. P-MTJ MRAM technology that usesspin-torque (STT-MRAM) for writing of memory bits was described by C.Slonczewski in “Current driven excitation of magnetic multilayers”, J.Magn. Magn. Mater. V 159, L1-L7 (1996), and is an increasingly promisingcandidate for future generations of non-volatile memory to replaceembedded flash memory and embedded cache memory (SRAM).

Both MRAM and STT-MRAM have a p-MTJ cell based on a tunnelingmagnetoresistance (TMR) effect wherein a stack of layers has aconfiguration in which two ferromagnetic layers are separated by a thininsulating tunnel barrier layer such as MgO. One of the ferromagneticlayers called the pinned layer has a magnetic moment that is fixed in anout-of-plane direction such as the +z direction when the plane of eachlayer is laid out in the x-axis and y-axis directions. The secondferromagnetic layer has an out-of-plane magnetization direction that isfree to rotate to either the +z-axis (parallel or P state) or the−z-axis (antiparallel or AP state) direction. The difference inresistance between the P state (Rp) and AP state (Rap) is characterizedby the equation (Rap−Rp)/Rp also known as DRR or the MR ratio. It isimportant for p-MTJ cells to have a large MR ratio, preferably higherthan 100%, as the MR ratio is directly related to the read margin forthe memory bit, or how easy it is to differentiate between the P stateand AP state (0 or 1 bits).

Another critical requirement for p-MTJs is thermal stability during 400°C. process temperatures that are typical of back-end-of-line (BEOL)processes when fabricating embedded memory devices in complementarymetal-oxide-semiconductor (CMOS) products. A general trend has been tointroduce a second metal oxide/free layer (FL) interface similar to thetunnel barrier/FL interface thereby enhancing PMA and Hk within the freelayer, and improving thermal stability. Unfortunately, a consequence ofemploying a Hk enhancing (cap) layer such as MgO on the free layer isthe addition of parasitic resistance to the p-MTJ. Equation (1) showsthe effect of the cap layer resistance contribution to total p-MTJresistance while Equation (2) indicates the impact on DRR (MR ratio).

$\begin{matrix}{{{DRR} = {\frac{R_{AP} - R_{P}}{R_{P}}\mspace{14mu} {where}}}{R_{AP} = {R_{AP}^{barrier} + {R_{AP}^{cap}\mspace{14mu} {and}}}}{R_{P} = {R_{P}^{barrier} + {R_{P}^{cap}\mspace{14mu} {Since}}}}{R_{AP}^{cap} = R_{P}^{cap}}} & {{Eq}.\mspace{11mu} (1)} \\{{DRR} = {\frac{R_{AP}^{barrier} + R_{AP}^{cap} - \left( {R_{P}^{barrier} + R_{P}^{cap}} \right)}{R_{P}^{barrier} + R_{P}^{cap}} = \frac{R_{AP}^{barrier} - R_{P}^{barrier}}{R_{P}^{barrier} + R_{P}^{cap}}}} & {{Eq}.\mspace{11mu} (2)}\end{matrix}$

In summary, the series resistance (R_(AP) ^(cap) and R_(P) ^(cap))caused by the metal oxide cap layer will cause a reduction in DRR,unfortunately reducing the MRAM bit reading margin as well as increasingthe bit's writing voltage by adding a series resistance. Therefore, analternative p-MTJ cell structure is needed that features a RA productpreferably less than 5 ohm-□m² for advanced memory products whileachieving an acceptable Hk (PMA) for enhanced thermal stability, andincreasing the MR ratio above 100% while minimizing switching voltage(Vc).

SUMMARY

One objective of the present disclosure is to reduce the switchingvoltage and resistance x area (RA) product for p-MTJ cells whileoptimizing Hk and the MR ratio to provide the overall magneticperformance necessary to satisfy requirements for advanced embeddedmemory products.

A second objective is to provide a method of forming the p-MTJ of thefirst objective that is readily implemented in embedded memory devicesfound in CMOS products.

According to one embodiment of the present disclosure, a p-MTJconfiguration is provided where a free layer is formed between a metaloxide tunnel barrier layer and a metal Hk enhancing layer such as W orMo in order to minimize the RA product, and increase the MR ratio byremoving the R_(P) ^(cap) contribution to equation (2) describedpreviously. The p-MTJ may have a bottom spin valve configuration withthe tunnel barrier layer below the free layer, or a top spin valveconfiguration with the free layer below the tunnel barrier layer.

The free layer may be a single layer as deposited and comprises at leastFe, Co, and B. In some embodiments, one or more additional elements (M)such as oxygen, nitrogen or a metal from an adjacent layer diffuse intothe free layer during p-MTJ fabrication to give a final free layercomposition that is (Fe_(x)Co_(y)B_(z))_(w)M_(100-w) where x is 66-80, yis 5-9, z is 15-28, x+y+z=100, and w >90 atomic %. Moreover, the freelayer thickness is preferably from 8 to 15 Angstroms to realize a smallswitching voltage (V_(C))<0.15V (direct current or DC) since Vc isproportional to the magnetization volume of the free layer. It isbelieved that a certain degree of segregation occurs during one or moreannealing steps after the MTJ stack of layers is formed so that thefinal p-MTJ structure may have a free layer with a FeB/FeCoB orFeCoB/FeB bilayer configuration, or even a FeB/FeCoB/FeB or FeB/FeCoB/Fetrilayer stack where a lower layer contacting the tunnel barrier has aFeB composition, a middle portion maintains a FeCoB composition, and anupper layer contacting the Hk enhancing layer in a bottom spin valveembodiment has a Fe or FeB composition. When one or more M elements arepresent, M may be unevenly dispersed within the free layer depending onthe atomic number of M and the bond strength of an M atom with the otheratoms in the free layer.

According to a second embodiment, a free layer has a trilayer stack asdeposited and is formed with a FeB/FeCoB/Fe or FeB/FeCoB/FeBconfiguration between a tunnel barrier and a Mo or W Hk enhancing layer.In other words, a first layer that contacts the tunnel barrier iscomprised of Fe and B. In particular, when the tunnel barrier is MgO, aMgO/FeB interface provides enhanced Hk compared with a MgO/CoFeBinterface, for example. However, a free layer consisting only of FeBdoes not meet the objectives of the present disclosure since a high MRratio is not realized simultaneously with enhanced Hk. Therefore, acertain amount of Co is employed in the iron rich FeCoB middle layer.Since Co has a lower affinity for oxygen than Fe, Co is advantageouslyused in the middle portion of the free layer to block oxygen migrationfrom the tunnel barrier layer to the third layer (Fe or FeB) therebymaintaining Hk that results from the third layer interface with the W orMo Hk enhancing layer. The addition of Co to a FeB alloy also enhancesthe MR ratio. Furthermore, the trilayer thickness is preferably from 8to 15 Angstroms so that Vc is minimized. A total composition of thetrilayer structure is (Fe_(x)Co_(y)B_(z)) where x is 66-80, y is 5-9, zis 15-28, and x+y+z is 100, or the (Fe_(x)Co_(y)B_(z))_(w)M_(100-w)composition described earlier wherein w >90 atomic % when one or more Melements are incorporated into the trilayer during p-MTJ fabrication.

Another key feature of the trilayer embodiment is that the third layerportion contacting the Mo or W Hk enhancing layer must be Fe or Fe-richmaterial to minimize or avoid a dead zone. Otherwise, for the case ofalloys such as CoFeB, there is intermixing with the Mo or W layer thatresults in a dead zone with substantially reduced PMA. Thus, a Fe orFe-rich FeB layer has essentially no intermixing with W or Mo andthereby provides a maximum Hk contribution to PMA within the free layerfrom the Fe/Hk enhancing layer interface or from the FeB/Hk enhancinglayer interface.

The present disclosure also encompasses a method of forming a free layerbetween a tunnel barrier and a W or Mo Hk enhancing layer wherein thefree layer is formed in multiple steps. In one embodiment, a first layercomprised of Fe and B is deposited on the tunnel barrier layer. Then, aFeCoB layer that comprises more Co than in the first layer is depositedto give a FeB/FeCoB stack, for example. Thereafter, a third layer isdeposited on the FeCoB layer and is FeB or Fe having a Fe content thatis greater than the Fe content in the FeCoB layer. After a W or Mo Hkenhancing layer is deposited on the free layer, and optionally a hardmask on the W or Mo layer, the p-MTJ stack is annealed at atemperature >380° C. The annealing may comprise one or more steps thatoccur before or after the p-MTJ stack of layers is patterned into aplurality of p-MTJ cells.

In an alternative embodiment, a Fe_(x)Co_(y)B_(z) single layer isdeposited on the tunnel barrier layer, and then a W or Mo Hk enhancinglayer is formed on the Fe_(x)Co_(y)B_(z) free layer. After an optionalhard mask is deposited, the resulting MTJ stack of layers is annealed ata temperature >380° C. thereby causing segregation in theFe_(x)Co_(y)B_(z) free layer to yield a free layer that is a FeB/FeCoBor FeCoB/FeB bilayer, or that is a trilayer with a FeB/FeCoB/Fe orFeB/FeCoB/FeB configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a p-MTJ wherein a free layer isformed between a tunnel barrier layer and a Mo or W Hk enhancing layerin a bottom spin valve configuration according to an embodiment of thepresent disclosure.

FIG. 2 is a cross-sectional view showing a p-MTJ wherein a free layer isformed between a Mo or W Hk enhancing layer and a tunnel barrier layerin a top spin valve configuration according to another embodiment of thepresent disclosure.

FIG. 3 depicts a p-MTJ previously fabricated by the inventors wherein aCoFeB layer is formed between a MgO tunnel barrier and a Mo cap layer,which results in an intermixing layer.

FIG. 4 is a cross-sectional view of a p-MTJ wherein the free layer is aCoFeB/FeB bilayer to substantially reduce or avoid the intermixing layerat an interface with an overlying Mo layer.

FIG. 5 is a cross-sectional view of a p-MTJ wherein a Fe_(x)Co_(y)B_(z)free layer is formed between a MgO tunnel barrier and a Mo Hk enhancinglayer, and may segregate to a FeCoB/FeB or FeB/FeCoB bilayer duringannealing according to a first embodiment of the present disclosure.

FIGS. 6A-6B are cross-sectional views of a p-MTJ with a bottom spinvalve and top spin valve configuration, respectively, wherein a freelayer with a FeB/FeCoB/Fe or FeB/FeCoB/FeB trilayer configuration and atotal composition equivalent to Fe_(x)Co_(y)B_(z) is formed between aMgO tunnel barrier and a Hk enhancing layer according to a secondembodiment of the present disclosure.

FIGS. 7-8 are phase diagrams for Co—Mo and Fe—Mo, respectively, thatindicate solubility of Co in Mo, and insolubility of Fe with Mo at CMOSprocess temperatures.

FIG. 9 is a bar chart that shows a comparison of free layer compositionspreviously fabricated by the inventors with embodiments of the presentdisclosure with regard to a figure of merit A where A is the product ofHk and MR ratio.

FIG. 10 is a ternary plot of Fe, Co, and B composition showing the freelayer composition region represented by the embodiments of the presentdisclosure, and outlying regions typical of the prior art.

FIGS. 11-13 show a sequence of steps used to fabricate a p-MTJ accordingto an embodiment of the present disclosure.

FIG. 14 is a flow diagram depicting a series of steps where aFe_(x)Co_(y)B_(z) free layer is formed between a tunnel barrier layerand Mo or W Hk enhancing layer in a p-MTJ stack of layers.

DETAILED DESCRIPTION

The present disclosure is related to p-MTJ cells and the fabricationthereof wherein thermal stability, MR ratio, RA product, and switchingvoltage are simultaneously optimized for embedded memory applications. Akey feature is a free layer comprised of Fe_(x)Co_(y)B_(z) where x is66-80, y is 5-9, and z is 15-28, and x+y+z=100, and the free layer formsa first interface with a tunnel barrier layer and a second interfacewith a Mo or W Hk enhancing layer in a p-MTJ. In some embodiments, oneor more elements M may diffuse into the free layer during p-MTJfabrication to give a (Fe_(x)Co_(y)B_(z))_(w)M_(100-w) composition wherew >90 atomic %. The p-MTJ may be incorporated in a MRAM, STT-MRAM, oranother spintronic device such as a spin torque oscillator (STO),sensor, or biosensor. Only one p-MTJ cell is depicted in the drawings,but typically millions of p-MTJ cells are arrayed in rows and columns ona substrate during fabrication of a memory device. A top surface for alayer is defined as a surface facing away from the substrate while abottom surface faces the substrate. An interface is a boundary regioncomprised of a bottom surface of one layer and an adjoining top surfaceof a second layer. A thickness of each layer is in the z-axis direction,and a plane (top or bottom surface) is laid out in the x-axis and y-axisdirections.

For advanced technology nodes, especially for MRAM and STT-MRAM cellshaving a critical dimension (CD)<60 nm, there is a difficult challengeto simultaneously satisfy important requirements including thermalstability to 400° C. process temperatures, MR ratio above 100%, RAproduct <5 ohm-□m², and switching voltage <0.15V (DC), and preferably<0.1V (DC). P-MTJ cell improvements in the prior art address two or atmost three of the aforementioned requirements but fail to satisfy allfour of these performance needs. The present disclosure provides asolution to meet all of the aforementioned requirements. However, thepresent disclosure is not limited to high-end memory devices and alsoprovides the simultaneous benefits of 400° C. thermal stability, MRratio >100%, and switching voltage <0.15V (DC) for applications where anRA product >5 ohm-□m² is acceptable.

In related U.S. Pat. No. 8,372,661, we disclosed a Fe/CoFeB/Fe trilayerconfiguration for a free layer that was designed to reduce switchingcurrent. Although MR ratios above 100% were achieved, the RA product wasfrom 8 to 10 ohm-□m² while thermal stability and Hk were not discussed.Also, in related U.S. Pat. No. 9,780,299, we disclosed that improvedseed layer stacks with a higher degree of uniformity (top surfacesmoothness) than previously realized translate to improved thermalstability at 400° C. However, other magnetic performance relatedparameters were not discussed.

Now we have found that all performance requirements mentioned earlierare achieved in a p-MTJ cell comprised of a tunnel barrier/free layer/Moor W Hk enhancing layer stack or with a Mo or W Hk enhancing layer/freelayer/tunnel barrier stack, and wherein the free layer comprisesFe_(x)Co_(y)B_(z) where x is 66-80, y is 5-9, z is 15-28, and x+y+z=100.

FIG. 1 depicts a patterned p-MTJ 1 according to a first embodiment ofthe present disclosure wherein an optional seed layer 11, pinned layer12, tunnel barrier layer 13, free layer 14, Hk enhancing layer 17, andhard mask 16 are sequentially formed on a top surface 10 t of asubstrate that is a bottom electrode 10 in a MRAM structure, forexample. Sidewall is on the p-MTJ cell is formed during a conventionalphotoresist patterning and etching sequence where a pattern formed in aphotoresist layer (not shown) on top surface 16 t of the hard mask isetch transferred through the underlying p-MTJ stack of layers asexplained later. Free layer magnetization 14 m is free to rotateparallel or anti-parallel to the pinned layer magnetization 12 m in thisbottom spin valve embodiment.

According to another embodiment of the present disclosure shown as p-MTJ2 in FIG. 2, a top spin valve configuration is illustrated where thep-MTJ layers are retained from FIG. 1 except they are stacked in adifferent order. Here, the optional seed layer 11, Mo or W Hk enhancinglayer 17, free layer 14, tunnel barrier 13, reference layer 12, and hardmask 16 are sequentially formed on a substrate comprised of an uppermostbottom electrode 10. The seed layer is comprised of one or more of NiCr,Ta, Ru, Ti, TaN, Cu, Mg, or other materials typically employed topromote a smooth and uniform grain structure in overlying layers. Inrelated U.S. Pat. No. 9,780,299, we described one of the preferred seedlayer configurations that is a TaN/Mg/CoFeB/NiCr stack of layers inorder to provide an especially smooth top surface 11 t that improvesmagnetic properties in overlying layers.

Referring to FIG. 3, we have previously fabricated p-MTJ cells comprisedof a MgO tunnel barrier 13, CoFeB free layer 14 x 1, and Mo Hk enhancinglayer 17 having a bcc crystal structure. Although the MgO/CoFeBinterface 40 generates PMA that induces magnetization 14 m 1, we observethere is intermixing between the CoFeB free layer (FL) and Mo Hkenhancing layer whereby a dead zone 20 is formed that essentiallyprevents PMA to be induced at the original FL/Mo interface 41. As aresult, thermal stability suffers because of the lack of a second Hkcontribution from interface 41 that is needed to enhance PMA in the freelayer.

Referring to FIG. 4, we have modified the p-MTJ cell in FIG. 3 toinclude a bilayer scheme to yield a free layer 14 x 2 where an upperlayer 14 b that is FeB contacts the Mo layer 17 at interface 41 tosubstantially reduce or prevent the intermixing region 20 a. Here, wefind an additional Hk contribution (magnetization 14 m 2) that togetherwith magnetization 14 m 1 at interface 40 provides enhanced PMA for thefree layer compared with the FL structure in FIG. 3. However, due to theunoptimized composition of the lower CoFeB layer 14 a, the MR ratio forfree layer 14 x 2 is generally too low to be acceptable for advancedMRAM and STT-MRAM applications.

To our knowledge, the role of the free layer composition at interface 41with a Mo or W Hk enhancing layer 17 (also known as a cap layer in abottom spin valve configuration) has not been previously addressed withregard to intermixing between FL 14 and the Hk enhancing layer. Inparticular, our observation that an uppermost Fe or FeB sub-layer in acomposite free layer 14 in FIG. 1 (or a bottommost Fe or FeB sub-layerin free layer 14 in FIG. 2) prevents undesirable intermixing with Mo orW in Hk enhancing layer 17 is a key feature of the present disclosure.In other words, optimum Hk at interface 41 and sufficient PMA in freelayer 14 to provide acceptable thermal stability at 400° C. processtemperatures is achieved with a Fe/Mo, Fe/W, FeB/Mo, or FeB/W interfaceaccording to embodiments described herein.

The intermixing behavior in FIG. 3 and lack thereof in FIG. 4 issupported by information found in phase diagrams in the Fact Sage steeldatabase. Referring to FIG. 7, a Co—Mo phase diagram indicatessolubility (intermixing) of Mo and Co at typical CMOS processtemperatures. On the other hand, in FIG. 8, a corresponding Fe—Mo phasediagram indicates essentially no intermixing of Fe with Mo at typicalCMOS process temperatures.

Returning to FIGS. 1-2, reference layer 12 may have a syntheticanti-parallel (SyAP) configuration represented by AP2/Ru/AP1 where ananti-ferromagnetic coupling layer made of Ru, Rh, or Ir, for example, issandwiched between an AP2 magnetic layer and an AP1 magnetic layer (notshown). The AP2 (outer pinned) layer is formed on the seed layer 11 inFIG. 1 or adjoins the hard mask 16 in FIG. 2. AP1 is the inner pinnedlayer and typically contacts the tunnel barrier. One or both of the AP1and AP2 layers may be comprised of CoFe, CoFeB, Fe, Co, or a combinationthereof. In other embodiments, one or both of the AP1 and AP2 layers maybe a laminated stack with inherent PMA such as (Co/Ni)_(n),(CoFe/Ni)_(n), (Co/NiFe)_(n), (Co/Pt)_(n), (Co/Pd)_(n), or the likewhere n is the lamination number. Furthermore, a dusting layer that isCo or a Co rich alloy may be inserted between the AFM coupling layer andeach of the AP1 and AP2 layers to yield an AP2/Co/Ru/Co/AP1 referencelayer configuration with enhanced PMA and thermal stability as wedisclosed in related U.S. Pat. No. 9,472,752.

Tunnel barrier layer 13 is preferably a metal oxide that is one of MgO,TiOx, AlTiO, MgZnO, Al₂O₃, ZnO, ZrOx, HfOx, or MgTaO, or a lamination ofone or more of the aforementioned metal oxides. More preferably, MgO isselected as the tunnel barrier layer because it provides the highest MRratio (DRR).

The Mo or W Hk enhancing layer 17 has a thickness from 10 to 50Angstroms, and preferably 20 to 30 Angstroms. In some embodiments, a MoWalloy may be used as the Hk enhancing layer, or a MoD alloy or WD alloywhere D is one of Nb, Ti, Ta, Zr, Hf, V, or Cr, and wherein the Dcontent is less than 20 atomic %.

Hard mask 16 is non-magnetic and generally comprised of one or moreconductive metals or alloys including but not limited to Ta, Ru, TaN,Ti, TiN, and W. It should be understood that other hard mask materialsincluding MnPt may be selected in order to provide high etch selectivityrelative to underlying MTJ layers during an etch process that forms MTJcells with sidewalls that stop on the bottom electrode. Moreover, thehard mask may comprise an electrically conductive oxide such as RuOx,ReOx, IrOx, MnOx, MoOx, TiOx, or FeOx.

According to a first embodiment of the present disclosure shown in FIG.5, free layer 14 has a Fe_(x)Co_(y)B_(z) composition as deposited wherex is 66-80, y is 5-9, z is 15-28, and x+y+z=100. However, after one ormore annealing steps during subsequent memory device fabrication, one ormore M elements where M is a metal, oxygen, or nitrogen from theunderlying tunnel barrier layer (or Hk enhancing layer or anotheradjacent layer) may diffuse into the FL to yield a(Fe_(x)Co_(y)B_(z))_(w)M_(100-w) composition where w >90 atomic %.Moreover, the Fe_(x)Co_(y)B_(z) free layer may segregate into two ormore sub-layers. Thus, the present disclosure anticipates that the freelayer in FIG. 1 or FIG. 2 may transform to a bilayer 14′ with aFeB/FeCoB or FeCoB/FeB configuration after annealing or exposure toprocess temperatures >380° C. Note that after segregation occurs, the“total” composition of the FL may be maintained as Fe_(x)Co_(y)B_(z). Hkcontribution 14 m 1 at interface 40 and Hk contribution 14 m 2 atinterface 41 both contribute to FL magnetization 14 m in FIG. 1 and FIG.2.

Referring to FIG. 6A, a second embodiment of the present disclosure isdepicted where free layer 14 has a trilayer configuration that is formedby sequentially depositing a FeB layer 14-1 on a tunnel barrier 13 suchas MgO in a bottom spin valve p-MTJ structure, a FeCoB middle layer14-2, and an uppermost Fe or FeB layer 14-3 where a total composition offree layer 14 is Fe_(x)Co_(y)B_(z) described previously. The Hkenhancing layer 17 such as Mo, W, or a Mo alloy or W alloy describedearlier is then deposited on the uppermost Fe or FeB layer followed by ahard mask 16 depicted in FIG. 1. In a top spin valve embodiment shown inFIG. 6B, the sequence is reversed where Fe or FeB layer 14-3, FeCoBmiddle layer 14-2, and FeB layer 14-1 are sequentially deposited on theHk enhancing layer. Thereafter, the tunnel barrier is formed on layer14-1 followed by the reference layer 12 and hard mask 16 as shown inFIG. 2. As in the previous embodiment, the FL may have a(Fe_(x)Co_(y)B_(z))_(w)M_(100-w) composition if one or more M elementsdiffuse into one or more layers 14-1, 14-2, 14-3 during subsequentprocessing steps at temperatures >380° C.

The benefits of the trilayer stack for free layer 14 shown in FIGS.6A-6B are that all four of the p-MTJ objectives of the presentdisclosure are simultaneously realized. In particular, when the tunnelbarrier 13 is MgO, an interface 40 with FeB layer 14-1 provides enhancedHk 14 m 1 compared with a MgO/CoFeB interface in FIG. 4, for example.However, a free layer consisting only of FeB does not meet theobjectives of the present disclosure since a high MR ratio is notrealized simultaneously with enhanced Hk. Therefore, a certain amount ofCo is employed in the iron rich FeCoB middle layer 14-2. Since Co has alower affinity for oxygen than Fe, Co is advantageously used in themiddle portion of the free layer to block oxygen migration from thetunnel barrier layer to the Fe or FeB layer 14-3 thereby maintaining Hk14 m 2 that results from interface 41 with the Hk enhancing layer 17.The addition of Co to a FeB alloy in middle layer 14-2 also enhances theMR ratio.

The thickness of free layer 14 is preferably from 8 to 15 Angstroms sothat Vc is minimized, and a total composition of the trilayer structureshould be equivalent to Fe_(x)Co_(y)B_(z) described previously. It isalso important that layer 14-3 contacting the Hk enhancing layer is Feor an Fe-rich material to minimize or avoid a dead zone havingsubstantially reduced PMA or no PMA that results when Co or a Co alloysuch as CoFeB intermixes with a W, Mo, Mo alloy, or W alloy layer. Thus,a Fe or Fe-rich FeB layer has essentially no intermixing with W, Mo, oralloys thereof and thereby provides a maximum Hk value resulting frominterface 41. An Fe rich FeB layer is defined as a layer with a Fecontent >50 atomic %. Preferably, the Fe content is >70 atomic %, andmore preferably is ≥90 atomic %.

To demonstrate the benefits of the p-MTJ configurations disclosedherein, we performed an experiment to compare p-MTJ configurationshaving various free layers (comparative examples) formed during our pastp-MTJ development projects with those formed according to theembodiments described herein. The base film structure employed for allp-MTJ stacks isTaN20/Mg7/CoFeB9/NiCr50/(Ni6/Co1.8)₃/Co5/Ru4/Co5/Mo2/CoFeB4/Fe5/MgO/freelayer12/Mo20/Ta15/Ru100 where the thickness of each layer is shown inAngstroms. Total free layer thickness in each example is 12 Angstroms.In the base film structure, TaN/Mg/CoFeB/NiCr is the seed layer,(Ni/Co)₃ is the AP2 layer, the Ru AFM coupling layer is sandwichedbetween two Co dusting layers, the first Mo layer is a Hk enhancinglayer for the overlying AP1 layer, CoFeB/Fe is the AP1 layer thatadjoins the MgO tunnel barrier, the second Mo layer is the Hk enhancinglayer for the FL, and Ta/Ru is the hard mask. The Mo layer next to AP1also serves as a bridge (crystal decoupling) between the underlying Codusting layer with a fcc(111) structure and overlying CoFeB with abcc(002) crystal structure.

After all p-MTJ layers were deposited, each stack was annealed at 400°C. for 140 minutes to confirm thermal tolerance to typical 400° C.processes. Actual RA product results for all examples are within 3-4ohm/□m² and thus satisfactory for advanced embedded memory productshaving a CD<60 nm. MR ratio and Hk were measured after annealing, and acomparison was made among stacks by using a figure of merit A where A isthe product of MR ratio and Hk (in arbitrary units). A relative merit of0 is given when A<1000, 1 is for A between 1000 and 1100, 2 is for Abetween 1100 and 1200, 3 is for A between 1200 and 1300, 4 is for Abetween 1300 and 1400, and 5 (best result) is for A above 1400.

With regard to Table 1, the most common single free layer composition inour earlier development efforts is Fe₆₀Co₂₀B₂₀ and listed as Comp. Ex.1, and a second popular composition is Fe₇₀Co₃₀ (Comp. Ex. 2). Both havean A result below 1000. However, a single layer composition Fe₇₇Co₇B₁₆according to Embodiment 1 improves performance significantly. Ourearlier bilayer configurations are listed as Fe8/Fe₆₀Co₂₀B₂₀4 andFe₆₀Co₂₀B₂₀4/Fe₈, or Comp. Ex. 3 and Comp. Ex. 4, respectively, and showno improvement over the single layer baseline results. On the otherhand, bilayer configurations according to Embodiment 1, which arerepresented as Fe₇₀B₃₀8/Fe₆₀Co₂₀B₂₀4, Fe₇₀B₃₀7/Fe₆₀Co₂₀B₂₀5, andFe₆₀Co₂₀B₂₀4/Fe₇₀B₃₀8 provide A results between 1000 and 1200, aconsiderable improvement over prior art bilayers.

TABLE 1 P-MTJ properties withTaN20/Mg7/CoFeB9/NiCr50/(Ni6/Co1.8)₃/Co5/Ru4/Co5/Mo2/CoFeB4/Fe5/MgO/free layer/Mo20/Ta15/Ru100 stack annealed at 400° C. for140 min. MR A = Free Layer FL Hk ratio Hk × MR Rel. (Total thickness =12 Angstroms) Total FeCoB type (kOe) (%) ratio merit Comp. Fe₆₀Co₂₀B₂₀12Fe₆₀Co₂₀B₂₀ 1LR 7.5 98 735 0 Ex. 1 Comp. Fe₇₀Co₃₀ Fe₇₀Co₃₀ 1LR 7.0 90630 0 Ex. 2 Emb. 1 Fe₇₇Co₇B₁₆12 Fe₇₇Co₇B₁₆ 1LR 9.5 115 1093 1 Comp.Fe8/Fe₆₀Co₂₀B₂₀4 Fe_(86.7)Co_(6.7)B_(6.7) 2LR 3.5 120 420 0 Ex. 3 Emb. 1Fe₇₀Co₃₀8/Fe₆₀Co₂₀B₂₀4 Fe_(66.7)Co_(6.7)B_(26.7) 2LR 9.6 108 1037 1 Emb.1 Fe₇₀Co₃₀7/Fe₆₀Co₂₀B₂₀5 Fe_(65.8)Co_(8.3)B_(25.8) 2LR 9.6 123 1181 2Comp. Fe₆₀Co₂₀B₂₀4/Fe8 Fe_(86.7)Co_(6.7)B_(6.7) 2LR 4.0 55 220 0 Ex. 4Emb. 1 Fe₆₀Co₂₀B₂₀4/Fe₇₀Co₃₀8 Fe_(66.7)Co_(6.7)B_(26.7) 2LR 9.2 130 11962 Comp. Fe4/Fe₆₀Co₂₀B₂₀4/Fe4 Fe₈₈Co₆B₆ 3LR 3.5 110 385 0 Ex. 5 Emb. 2Fe₇₀B₃₀4/Fe₆₀Co₂₀B₂₀4/Fe4 Fe_(76.7)Co_(6.7)B_(16.7) 3LR 9.4 139 1307 4Emb. 2 Fe₇₀B₃₀4/Fe₆₀Co₂₀B₂₀4/Fe₉₀B₁₀4 Fe_(73.3)Co_(6.7)B₂₀ 3LR 9.3 1371274 3 Emb. 2 Fe₇₀B₃₀4/Fe₆₀Co₂₀B₂₀4/Fe₈₅B₁₅4 Fe_(71.7)Co_(6.7)B_(21.7)3LR 9.2 130 1196 2 Emb. 2 Fe₇₀B₃₀4/Fe₆₀Co₂₀B₂₀4/Fe₈₀B₂₀4Fe₇₀Co_(6.7)B_(23.3) 3LR 9.0 131 1179 2 Emb. 2Fe₇₀B₃₀4/Fe₆₀Co₂₀B₂₀4/Fe₇₀B₃₀4 Fe_(66.7)Co_(6.7)B_(26.7) 3LR 9.0 1331197 2 Comp. Fe4/Fe₆₀Co₂₀B₂₀4.6/Fe3.4 Fe_(84.7)Co_(7.7)B_(7.7) 3LR 3.5110 385 0 Ex. 6 Emb. 2 Fe₇₀B₃₀4/Fe₆₀Co₂₀B₂₀4.6/Fe3.4Fe_(74.7)Co_(7.7)B_(17.7) 3LR 10.4 136 1414 5 Emb. 2Fe₇₀B₃₀4/Fe₆₀Co₂₀B₂₀4.6/Fe₇₀B₃₀3.4 Fe_(66.2)Co_(7.7)B_(26.2) 3LR 9.0 1301170 2 Comp. Fe4/Fe₆₀Co₂₀B₂₀3.4/Fe4.6 Fe_(88.7)Co_(5.7)B_(5.7) 3LR 4.592 414 0 Ex. 7 Emb. 2 Fe₇₀B₃₀4/Fe₆₀Co₂₀B₂₀3.4/Fe4.6Fe_(78.7)Co_(5.7)B_(15.7) 3LR 9.0 133 1197 2 Emb. 2Fe₇₀B₃₀4/Fe₆₀Co₂₀B₂₀3.4/Fe₇₀B₃₀4.6 Fe_(67.2)Co_(5.7)B_(27.2) 3LR 9.6 1241190 2

Trilayer configurations from our prior p-MTJ development studies arelisted as Fe4/Fe₆₀Co₂₀B₂₀4/Fe4 (Comp. Ex. 5), Fe4/Fe₆₀Co₂₀B₂₀4.6/Fe3.4(Comp. Ex. 6), and Fe4/Fe₆₀Co₂₀B₂₀3.4/Fe4.6 (Comp. Ex. 7). Again, thereis no improvement over the baseline results in Comp. Ex. 1 and 2. Incomparison, all Embodiment 2 trilayer examples have A results above1000, and in several cases, have an A value substantially higher thanthe Embodiment 1 (single layer or bilayer) configurations. For instance,the Fe₇₀B₃₀4/Fe₆₀Co₂₀B₂₀4/Fe4 and Fe₇₀B₃₀4/Fe₆₀Co₂₀B₂₀4.6/Fe3.4 examplesaccording to the present disclosure have the best A results of 1307 and1414, respectively. The latter is almost 2× higher than the Comp. Ex. 1baseline value of 735. The Fe₇₀B₃₀4/Fe₆₀Co₂₀B₂₀4/Fe₉₀B₁₀4 trilayer alsoprovides enhanced A results above 1200.

The results from Table 1 are displayed in bar chart form in FIG. 9. Insummary, single layer or bilayer configurations having aFe_(x)Co_(y)B_(z) total free layer composition where x is 66-80, y is5-9, z is 15-28, and x+y+z=100 provide A results between 1000 and 1200,but the most desirable A results (1170-1414) are generated by trilayerswith the Fe_(x)Co_(y)B_(z) composition. All compositions shown in Table1 and in FIG. 9 are free layers as deposited and do not account for anyM elements that may have diffused into the free layer during p-MTJfabrication.

FIG. 10 is a FeCoB alloy graph that shows the preferred region Acomprised of solid circles for achieving RA product <5 ohm/□m², MRratio >100%, and thermal stability to 400° C. Note that switchingvoltage was not measured since the p-MTJ stacks were not patterned.However, Vc is expected to be <0.15V (DC) in view of p-MTJ stacks with atotal FL composition of Fe_(x)Co_(y)B_(z) that have been patterned andcharacterized in other experiments that are not reported here. Outlyingregions with open circles contain comparative example configurationsshown in Table 1. In review, the comparative examples fail to providedesirable A values because Hk is degraded due to free layer intermixingwith the Mo Hk enhancing layer. In general, MR ratio is below 100% inthe outlying regions to further reduce the A result.

As mentioned earlier, free layer 14 in FIG. 1 and FIG. 2 may have athickness from 8 to 15 Angstroms in either a single layer (as deposited)or triple layer configuration. Table 2 shows possible thicknesses ofeach layer in a Fe₇₀B₃₀/Fe₆₀Co₂₀B₂₀/Fe trilayer configuration thatconforms to the Fe_(x)Co_(y)B_(z) total free layer composition accordingto an embodiment of the present disclosure.

TABLE 2 Trilayer stacks with thickness from 8 to 15 Angstroms, and totalFL composition Fe_(x)Co_(y)B_(z) where x = 66-80, y = 5-9, z = 15-28 andx + y + z = 100. Total FL thickness FL 14-1 FL 14-2 FL 14-3Fe_(x)Co_(y)B_(z) 15 Angstroms Fe₇₀B₃₀ 5 Fe₆₀Co₂₀B₂₀ 5 Fe 5Fe_(76.7)Co_(6.7)B_(16.7) 14 Angstroms Fe₇₀B₃₀ 5 Fe₆₀Co₂₀B₂₀ 5 Fe 4Fe_(75.0)Co_(7.1)B_(17.9) 13 Angstroms Fe₇₀B₃₀ 4 Fe₆₀Co₂₀B₂₀ 5 Fe 4Fe_(75.4)Co_(7.7)B_(16.9) 12 Angstroms Fe₇₀B₃₀ 3.5 Fe₆₀Co₂₀B₂₀ 4.5 Fe 4Fe_(76.3)Co_(7.5)B_(16.3) 11 Angstroms Fe₇₀B₃₀ 3.5 Fe₆₀Co₂₀B₂₀ 4 Fe 3.5Fe_(75.9)Co_(7.3)B_(16.8) 10 Angstroms Fe₇₀B₃₀ 3.5 Fe₆₀Co₂₀B₂₀ 3.5 Fe 3Fe_(75.5)Co_(7.0)B_(17.5)  9 Angstroms Fe₇₀B₃₀ 3 Fe₆₀Co₂₀B₂₀ 3 Fe 3Fe_(76.7)Co_(6.7)B_(16.7)  8 Angstroms Fe₇₀B₃₀ 2.5 Fe₆₀Co₂₀B₂₀ 3 Fe 2.5Fe_(75.8)Co_(7.5)B_(16.9)Table 3 shows possible thicknesses of each layer in aFe₇₀B₃₀/Fe₆₀Co₂₀B₂₀/Fe₇₀B₃₀ trilayer configuration that conforms to theFe_(x)Co_(y)B_(z) total free layer composition according to anembodiment of the present disclosure.

TABLE 3 Trilayer stacks with thickness from 8 to 15 Angstroms, and totalFL composition Fe_(x)Co_(y)B_(z) where x = 66-80, y = 5-9, z = 15-28 andx + y + z = 100. Total FL thickness FL 14-1 FL 14-2 FL 14-3Fe_(x)Co_(y)B_(z) 15 Angstroms Fe₇₀B₃₀ 5 Fe₆₀Co₂₀B₂₀ 5 Fe₇₀B₃₀ 5Fe_(66.7)Co_(6.7)B_(26.7) 14 Angstroms Fe₇₀B₃₀ 5 Fe₆₀Co₂₀B₂₀ 5 Fe₇₀B₃₀ 4Fe_(66.4)Co_(7.1)B_(26.4) 13 Angstroms Fe₇₀B₃₀ 4 Fe₆₀Co₂₀B₂₀ 5 Fe₇₀B₃₀ 4Fe_(66.2)Co_(7.7)B_(26.2) 12 Angstroms Fe₇₀B₃₀ 3.5 Fe₆₀Co₂₀B₂₀ 4.5Fe₇₀B₃₀ 4 Fe_(66.3)Co_(7.5)B_(26.3) 11 Angstroms Fe₇₀B₃₀ 3.5 Fe₆₀Co₂₀B₂₀4 Fe₇₀B₃₀ 3.5 Fe_(66.4)Co_(7.3)B_(26.4) 10 Angstroms Fe₇₀B₃₀ 3.5Fe₆₀Co₂₀B₂₀ 3.5 Fe₇₀B₃₀ 3 Fe_(66.5)Co_(7.0)B_(26.5)  9 Angstroms Fe₇₀B₃₀3 Fe₆₀Co₂₀B₂₀ 3 Fe₇₀B₃₀ 3 Fe_(66.7)Co_(6.7)B_(26.7)  8 Angstroms Fe₇₀B₃₀2.5 Fe₆₀Co₂₀B₂₀ 3 Fe₇₀B₃₀ 2.5 Fe_(66.3)Co_(7.5)B_(26.3)

A sequence of steps in FIGS. 11-13 show one process flow for forming ap-MTJ cell having a free layer 14 formed between a tunnel barrier layer13, and a Hk enhancing layer 17 that is W, Mo, Mo alloy or a W alloy.Referring to FIG. 11, seed layer 11, reference layer 12, and tunnelbarrier 13 are sequentially formed on bottom electrode 10 byconventional methods. Thereafter, free layer 14 having top surface 14 tis formed with sequential deposition of a FeB layer 14-1, FeCoB layer14-2, and Fe or FeB layer 14-3 explained previously. All free layers aresputter deposited within a chamber having a base pressure of 5×10⁻⁸ to5×10⁻⁹ torr, and typically using a sputter gas that is Ar. Preferably,layer 14-2 comprises more Co (higher Co content) than in layer 14-1 orin layer 14-3. Moreover, layer 14-3 preferably comprises a higher Fecontent than in layer 14-2, and a B content ≤30 atomic %.

Referring to FIG. 12, a Hk enhancing layer 17 made of Mo, W, W alloy, ora Mo alloy may be sputter deposited on top surface 14 t in the samechamber used for depositing free layer 14. Thereafter, hard mask 16 withtop surface 16 t is deposited on the Hk enhancing layer.

Referring to FIG. 13, a photoresist layer is coated on top surface 16 tand patterned by a conventional photolithography technique to yield apattern comprised of island 30 with sidewall 30 s and a criticaldimension (CD)=d in the y-axis direction. Note that a dielectricanti-reflective coating (DARC) or bottom anti-reflective coating (BARC)known in the art is generally formed between top surface 16 t and thephotoresist layer to improve the process window and CD uniformity in aplurality of islands (not shown) across the substrate (wafer) but is notshown in this illustration. In some embodiments, the CD is also formedin the x-axis direction to give a circular island shape from a top-downview. However, the CD in the x-axis direction may be different from d sothat the island has an elliptical or polygonal shape from a top-downperspective.

One or more reactive ion etch (RIE) steps are employed to transfer thephotoresist sidewall through the p-MTJ layers, and stop on top surface10 t of the bottom electrode 10 as depicted in FIG. 1. As a result,sidewall 1 s on p-MTJ 1 is formed as a extension of sidewall 30 s. Thephotoresist island is removed during the etch transfer and hard mask 16ensures that the shape integrity (CD) of the original island shape 30 ispreserved in the resulting p-MTJ cell. An ion beam etch may be used toremove RIE residue before a subsequent encapsulation layer (not shown)is deposited to insulate p-MTJ 1 from adjacent p-MTJ cells. A firstanneal at >380° C. may be performed before the photoresist is coated andpatterned. Furthermore, one or more processing steps including annealsat temperatures >380° C. may occur after p-MTJ 1 is formed according tovarious embodiments of the present disclosure. It is believed that thefree layer will become crystalline after annealing in all embodimentsconsidering the iron rich content of all layers in single layer,bilayer, and trilayer configurations described herein. Accordingly, thecrystalline free layer should enable good lattice matching with a MgOtunnel barrier layer and with the Hk enhancing layer, which both have abody centered cubic (bcc) crystal structure after annealing.

Referring to FIG. 14, a process flow diagram is provided for analternative embodiment where a single free layer with aFe_(x)Co_(y)B_(z) composition where x is 66-80, y is 5-9, z is 15-28,and x+y+z=100 is formed between a tunnel barrier layer and a Hkenhancing layer. In the exemplary embodiment that relates to formationof a p-MTJ stack of layers with a bottom spin valve configuration, ap-MTJ stack of layers is provided with an uppermost tunnel barrier layerin step 70. Thereafter, in step 71, a Fe_(x)Co_(y)B_(z) free layer isdeposited on the tunnel barrier layer. An Hk enhancing layer such as W,Mo, W alloy, or a Mo alloy, and an optional hard mask are sequentiallyformed on the free layer in step 72. Finally, the p-MTJ stack isannealed at a temperature >380° C. in step 73 so that the free layer maysegregate into multiple sub-layers, and may absorb one or more Melements described previously. Alternatively, the sequence of steps isreversed so that the free layer is deposited on the Hk enhancing layer,and then the tunnel barrier is formed on the free layer when forming atop spin valve configuration. All of the aforementioned steps may becarried out within various chambers of a sputter deposition tool.

All of the embodiments described herein may be incorporated in amanufacturing scheme with standard tools and processes. P-MTJ cellsformed according to preferred embodiments disclosed herein have a RAproduct below 5 ohm/□m², MR ratio >100%, Vc<0.15V (DC), and sufficientHk to provide thermal stability to 400° C. process temperatures. Webelieve overall p-MTJ performance is an improvement over the prior artand thereby enables higher process yields of advanced product nodes such64 Mb and 256 Mb STT-MRAM technology, and related spintronic deviceswhere switching current, RA value, MR ratio, and thermal stability areall critical parameters.

While the present disclosure has been particularly shown and describedwith reference to, the preferred embodiment thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be made without departing from the spirit and scope of thisdisclosure.

What is claimed is:
 1. A method of forming a perpendicular magnetizedmagnetic tunnel junction (p-MTJ) stack, the method comprising: forming areference layer on a substrate; forming a tunnel barrier layer on thereference layer; forming a free layer on the tunnel barrier layer,wherein the free layer includes at least Fe, Co, and B in a compositionthat is Fe_(x)Co_(y)B_(z) where x is 66-80, y is 5-9, z is 15-28, andx+y+z=100, and wherein the free layer is formed by a process thatincludes: depositing a first layer that on the tunnel barrier layer, thefirst layer including FeB; depositing a second layer on the first layer,wherein the second layer includes FeCoB and has a higher Co content thanthe first layer; and depositing a third layer on the second, whereinthird layer includes Fe and has a higher Fe content than the secondlayer; depositing a Hk enhancing layer on the free layer, afterdepositing the Hk enhancing layer, performing an annealing process at atemperature greater than 380° C.
 2. The method of claim 1, whereindepositing the Hk enhancing layer on the free layer includes depositingthe Hk enhancing layer directly on the third layer.
 3. The method ofclaim 1, wherein the free layer has a thickness from about 8 to 15Angstroms, and wherein the Hk enhancing layer has a thickness from about10 to 50 Angstroms.
 4. The method of claim 1, wherein all layers in thep-MTJ stack are formed in one sputter deposition tool.
 5. The method ofclaim 1, wherein the third layer includes FeB.
 6. The method of claim 1,wherein the free layer further includes one or more M elements afterperforming the annealing process, and wherein M is oxygen, nitrogen, ora metal that diffused into the free layer to give a free layercomposition that is (Fe_(x)Co_(y)B_(z))_(w)M_(100-w) where w is >90atomic %.
 7. The method of claim 1, wherein the Hk enhancing layerincludes a material selected from the group consisting of W, Mo, a Walloy and a Mo alloy.
 8. A method comprising: forming a Hk enhancinglayer on a substrate; forming a free layer on the Hk enhancing layer,wherein the free layer includes at least Co, Fe, and B in a totalcomposition that is Fe_(x)Co_(y)B_(z) in which x is 66-80, y is 5-9, zis 15-28, and x+y+z=100, and wherein the free layer is formed by aprocess that includes: depositing a first layer that includes Fe or FeBon the Hk enhancing layer; depositing a second layer that includes FeCoBon the first layer, wherein the first layer has a higher Fe content thanthe second layer; and depositing a third layer that includes FeB on thesecond layer, wherein the second layer has a higher Co content than thefirst layer; and forming a tunnel barrier layer on the free layer; andforming a reference layer on the tunnel barrier layer.
 9. The method ofclaim 8, further comprising: after forming the reference layer on thetunnel barrier layer, performing an annealing process at a temperaturegreater than 380° C.; and patterning the reference layer afterperforming the annealing process.
 10. The method of claim 8, furthercomprising: patterning the reference layer; and after patterning thereference layer, performing an annealing process at a temperaturegreater than 380° C.
 11. The method of claim 8, wherein the free layerhas a thickness from about 8 to 15 Angstroms, and wherein the Hkenhancing layer has a thickness from about 10 to 50 Angstroms.
 12. Themethod of claim 8, wherein the Hk enhancing layer includes a materialselected from the group consisting of W, Mo, a W alloy and a Mo alloy,and wherein the tunnel barrier layer includes MgO.
 13. The method ofclaim 8, wherein the free layer physically contacts the Hk enhancinglayer and the tunnel barrier layer.
 14. The method of claim 8, furthercomprising performing an annealing process on the reference layer, andwherein the free layer further includes one or more M elements afterperforming the annealing process on the reference layer, and wherein Mis oxygen, nitrogen, or a metal that diffused into the free layer togive a free layer composition that is (Fe_(x)Co_(y)B_(z))_(w)M_(100-w)where w is >90 atomic %.
 15. A method of forming a perpendicularmagnetized magnetic tunnel junction (p-MTJ) device, the methodcomprising: providing a stack of layers that includes a reference layer,a Hk enhancing layer, a tunnel barrier layer and a free layer, whereinthe free layer is comprised of at least Fe, Co, and B in a compositionthat is Fe_(x)Co_(y)B_(z) in which x is 66-80, y is 5-9, z is 15-28, andx+y+z=100; and performing annealing process on the stack of layers. 16.The method of claim 15, wherein the annealing process is performed at atemperature greater than 380° C.
 17. The method of claim 15, wherein theannealing process causes the diffusion of one or M elements into thefree layer such that the free layer has a different composition afterthe performing of the annealing process, and wherein M is oxygen,nitrogen, or a metal.
 18. The method of claim 15, wherein the providingof the stack of layers includes providing: a seed layer disposed over asubstrate; the reference layer disposed over the seed layer, the tunnelbarrier layer disposed over the reference layer, the free layer disposedover the tunnel barrier layer, and the Hk enhancing layer disposed overthe free layer.
 19. The method of claim 15, wherein the providing of thestack of layers includes providing: a seed layer disposed over asubstrate; the Hk enhancing layer disposed over the seed layer, the freelayer disposed over the Hk enhancing layer, the tunnel barrier layerdisposed over the free layer, and the reference layer disposed over thetunnel barrier layer.
 20. The method of claim 15, wherein the free layeris formed of more than one material layer.